1. Field of the Invention
The present invention relates to a method and system which uses a MEMS chip wireless capacitive sensor containing an inductance coil and spaced apart capacitive plates, one of which is in pressure contact with a fluid. The sensor can be used in general applications and in animals, including humans, to sense pressure, particularly in the eye. In particular, the sensor is in contact with the fluid in the vitreous chamber adjacent the cornea or in the aqueous chamber adjacent to the cornea of the eye. The sensor preferably includes an external antenna.
2. Description of Related Art
The problems in monitoring eye pressure are exemplary of the general problems of pressure measurement in a living animal.
Intraocular Pressure In the Eye
Glaucoma is one of the most menacing diseases of the eye that exists today. Patients may suffer significant eye damage, including blindness, without experiencing a noticeable amount of pain or discomfort. Glaucoma is the second leading cause of blindness in the United States and is the leading cause of blindness among African Americans. There are several types of glaucoma; the most common of which is called primary open-angle glaucoma (POAG). POAG affects more than 3 million people with an additional 3-6 million Americans considered to be susceptible because they have one or more of the risk factors associated with the disease.
Glaucoma is a progressive disease that is characterized by a specific pattern of damage to the optic nerve. Development of the disease can be attributed to many risk factors, including but not limited to high intraocular pressure (IOP), a family history of glaucoma, myopia, blood pressure, and diabetes. Age also is a critical factor (Hart, W. M., Jr., “The epidemiology of primary open-angle glaucoma and ocular hypertension”, in R. Ritch, M. B. Shields, T. Krupin (eds): The Glaucomas. St. Louis, C. V. Mosby Co., pp 789-795 (1989)). The incidence of glaucoma increases approximately 10-fold between 50 and 70 years of age, ranging from about 0.2% of the population between the ages of 50 and 54 to 2.0% of the population aged 70-74. Also, primates with experimentally induced elevations of IOP show structure (Quigley, H. A. “Pathophysiology of the optic nerve in glaucoma”. In: J. A. McAllister, R. P. Wilson, eds. Glaucoma. London: Butterworths; 30-53 (1986); Morrison, J. C., et al., “A rat model of chronic pressure-induced optic nerve damage”. Exp. Eye Res. 64:85-96 (1997); Garcia-Valenzuela, E., et al., Exp Eye Research. 61:33-44 (1995); and John, S. W., et al., Invest Ophthalmol Vis Sci. 39:951-962 (1998)) and functional (Marx, M. S., et al., “The pattern ERG and VEP in glaucomatous optic nerve disease in the monkey and human”, in R. Q. Cracco, I. Bodis-Wollner (eds): Evoked Potentials. New York, Alan R. Liss, Inc., pp 117-126 (1986); Marx., M. S., et al., Doc Ophthalmol 67:281-301 (1988(a)); Frishman, L. J., et al., Invest Ophthalmol Vis Sci. 37:125-141 (1996); and Harwerth, R. S., et al., Invest Ophthalmol Vis Sci. 40:2242-2250 (1999)) changes that are characteristic of humans with POAG, so implantation into primates for testing precedes implantation into humans.
While it is important to note that elevated IOP is neither synonymous with glaucoma nor a guaranteed predictor of disease, it remains the most important risk factor related to the disease. IPO can be associated with much of the development and progression of glaucoma damage that occurs through time. Patients with unilateral elevation of intraocular pressure that is secondary to other eye disorders often develop glaucoma.
In the normal eye (see FIG. 1), IOP is maintained at ˜16 millimeters of Mercury (mmHg) by a balance in the production and drainage of aqueous humor from the anterior chambers of the eye. This clear, blood-derived fluid flows from the ciliary body through the pupil to the Schlemm's canal. The aqueous humor is then discharged through the venous system into the vascular sclera by passing through the trabecular meshwork, a sponge-like structure located in the anterior angle of the eye (Kobayashi, A. S., Biomechanics of Medical Devices, Marcel Dekker, Inc., New York, New York (1981)). If the balance of the rate of production of aqueous humor and the rate of discharge is changed, the IOP is affected.
A patient's IOP experiences cyclical change on a daily basis. Fluctuations in IOP around the average value occur due to every day activity and changes in environment. IOP can show many different patterns of changes throughout a day that include impulses, prolonged periods of high pressure, or periods of sub-normal pressure without the patient's knowledge (Puers, R., et al., J. Micromech. Microeng., 10, pp. 124-129 (2000))
Measuring and monitoring of IOP is crucial for the diagnosis, treatment, management, and research of the disease. At the present time, the most common method for measuring a patient's IOP is a procedure called tonometry (Kobayashi, A. S., Biomechanics of Medical Devices, Marcel Dekker, Inc., New York, New York (1981)). While this method is considered to be very accurate for measuring IOP, there are several drawbacks. Only a single reading for the particular instant in time that the test is performed is possible. Also, a visit to a physician's office is generally required, thus individual measurements may be separated by long periods of time. Permanent damage to many parts of the eye, including the optic nerve and retina, can result within hours of the onset if the pressures are high enough. It is critical that IOP levels be monitored on a continuous basis so that pressure relieving drugs can be administered immediately after the onset of high pressures to minimize the risk of permanent damage.
An improved monitoring system would provide benefits in both clinical and research applications. Clinically, the primary targets for such a device would be patients with severe cases of glaucoma. From a research standpoint, there are many questions that are unanswered about the true effects of IOP. Doctors still do not know what the largest concern is: the peak pressure over 24 hours, the difference between the high and low pressure measurements for a day, the cumulative IOP over a period of time, or an average IOP level. It is quite possible that one or all of these factors play a significant role in the progression of glaucoma. In this regard, such a device could lead to an extensive gain in knowledge of glaucoma and better methods of treatment.
Sensors
Wireless capacitive pressure sensors are known and described in the Collins, IEEE Transactions on Bio-Medical Engineering, Vol. BME-14 No. 2 74-83 (April 1967); Rosengren, et al., J. Micromech. Microeng. 2 202-204 (1992); and Mokwa, et al., Proc. Microsystem Symposium Delft, 1-13 (1998)). Akar et al, Eurosensors XIV Aug. 27-30, 2000 describes such a device for general pressure measurement. This type of sensor are also described by Puers et al in J. Microeng 10 124-129 (2000). U.S. Pat. No. 5,005,577 to Frenkel describes a device which is implantable in the aqueous chamber of the eye. Jeffries et al, U.S. Pat. No. 6,193,656 also describes such a system. Waters et al, U.S. Pat. No. 4,922,913 describes a piezoelectric sensor. U.S. Pat. No. 6,213,943 to Abreu describes a radio wave transmitter connected to the eye. U.S. Pat. No. 6,312,380 to Hoek et al and U.S. Pat. No. 4,026,276 to Chubbuck describe capacitive sensors used in human systems.
There is a need for an improved method and system for monitoring fluid pressures in an animal, particularly in the eye of a mammal, such as a human.